Levels of Lipoprotein (a), a lipoprotein closely related to LDL, are directly correlated with risks of heart attacks. Recent evidence suggests estrogen may lower Lp(a) levels. We plan to conduct placebo controlled, crossover design trial to determine the effects of estrogen replacement therapy in postmenopausal females on the levels of Lp(a). We will also use stable isotope methods to measure rates of production of Lp(a) in these women on/off estrogen.